I thought if we are going for 'creating their own' direction. Then here are some ideas that I found from The Crafty Crow.
First of all, I thought we could outline that space into sections (organs).
It would be nice to see the kids to fill the space themselves.
Then I thought of what kind of things we can use for them to fill the empty space.
Here are some examples of methods:
over lapsing objects to create movement
Rolling printing to create backgrounds